Background Cannabis is the second most commonly used substance after alcohol among people seeking treatment for other drug use, but no statistics are available regarding secondary cannabis use among drug treatment clients. Objectives To investigate levels of secondary cannabis use among drug treatment clients and perceived need for support addressing this use among clients and staff. Methods Cross-sectional surveys of clients (N=295) and staff (N=33) were conducted in 2015 at four London drug and alcohol treatment services. Client measures included recent drug use, type of cannabis used, Severity of Dependence Scale for cannabis, and views on secondary cannabis use treatment. Staff measures included definition of problem cannabis use, importance and timing for addressing secondary cannabis use. Results Amongst clients, 39.7% reported recent secondary cannabis use, with 30.8% of these clients meeting criteria for problem use. Problem users were more likely to be interested in receiving treatment for cannabis use than non-problem users (51.4% vs 10.8%, p<0.001). Nearly half of staff (48.5%) thought secondary cannabis use should be addressed early in treatment. Conclusions Two out of five drug treatment clients used cannabis and a third experienced cannabis-related problems. Many are willing to address cannabis use, but defined treatment pathways are needed.